1. Technical Field
This invention relates to prosthetic devices for the replacement of a tendon or a ligament.
2. Description of the Related Art
The structural integrity of movable joints within the body is provided by tough strands of connective tissue known as ligaments. If these structures are severely damaged through injury, surgical replacement by a prosthesis may be indicated. Biomaterials that have been used as artificial ligaments (e.g., U.S. Pat. No. 4,411,827 for a resorbable version, U.S. Pat. No. 4,209,859 for a permanent version.), methods of attaching prostheses (e.g., U.S. Pat. No. 4,246,660) and suitable surgical techniques have all been described, and such replacements are now considered routine surgical procedures.
Current surgical technique requires that the tension on the ligament be properly adjusted during the procedure. Undertensioning the prosthesis compromises the stability of the joint, whereas overtensioning restricts the range of movement of the joint and may damage the soft tissues on the abutting surfaces. Unfortunately, achieving the proper adjustment depends in large part on the skill of the surgeon. Little in the way of mechanical aids is available to assist in achieving the proper balance of force. One mechanical aid in tensioning a prosthetic ligament is disclosed in U.S. Pat. No. 4,712,542. Here, a prosthetic ligament is attached to one fixation site and extended to the other fixation site, where it is attached to a tension-sensing device. Tension on the prosthesis is adjusted until a predetermined tension is achieved throughout the range of motion of the joint.
The patent literature describes several methods of adjusting a prosthetic ligament, but such adjustment methods generally involve loosening or otherwise adjusting a bone screw or like device which has an interface with the bone. It is well known that repeated movement of such devices that contact the bone causes additional trauma and compromises the strength of the bone.
A device of this nature is disclosed in U.S. Pat. No. 4,246,660. This device comprises an adjustable connector with an upper plate connected to the prosthesis and a base plate connected to the bone. The plates have complementary toothed surfaces which can be made to mesh at the point where the prosthetic ligament is at the proper tension. Adjustment of tension, however, involves loosening a bone screw which passes through both plates and into the bone.
Described in U.S. Pat. No. 3,953,896 is a prosthetic ligament wherein both ends are capable of adjustment. The ends are threaded and adapted to engage complementary female threads in conical fastening end members. These end members in turn are adapted to engage conical bores in the bone. Adjustment is accomplished through turning the end members on the threaded ligament and subsequent fixing with bone cement.
A like device is described in U.S. Pat. No. 4,744,793. Here both ends of the prosthetic ligament are tubular, and the conical bores in the bone are fitted with conical hollow anchor plugs. The ligament is placed through the anchor plugs and fixed at one end with a locking plug, which is pressed into the tubular end portion of the prosthesis causing a friction fit between the prosthesis and the anchor plug. The other end of the prosthesis is then pulled to the desired tension and similarly fixed with a locking plug. Any readjustment of tension requires release of fixation of the prosthesis.
A device described in U.S. Pat. No. 3,896,500 addresses the adjustability problem by providing an anchor plate with teeth directed such that the artificial ligament may be pulled tighter without disturbing the fixation to bone. However, adjustment in this device is only one-way and no provision is made for loosening the ligament in the event of over-tensioning.